In a distributed redundant array of independent disks (Redundant Array of Independent Disks, RAID for short), a single RAID may be distributed in discrete spaces of multiple physical hard disks, so that hard disks in the system are fully used and hot spots are equally allocated to as many hard disks as possible, and the hot spots are eliminated. In addition, when a hard disk is damaged, more hard disks participate in reconstruction, so that a long reconstruction time is required. Therefore, the distributed RAID system is applied more and more widely. However, when an enterprise purchases and uses a storage system, the enterprise does not initially install a hard disk with maximum configurations, but purchases and adds hard disks according to services. For this reason, a capacity expansion issue of a distributed hard disk storage system needs to be considered.
To solve the capacity expansion issue of a hard disk, in the prior art, an original distributed RAID system is kept unchanged during the capacity expansion, and a new distributed RAID system is constructed in an added hard disk. However, this method for capacity expansion of the hard disk requires addition of many hard disks at a single time to construct a distributed RAID system with a short reconstruction time and disperse hot spots. In addition, after the capacity expansion, an original RAID group cannot be horizontally extended to an added RAID group, and the time of reconstructing the original RAID group cannot be reduced. Further, during the capacity expansion in the prior art, added hard disks are added to the original RAID group respectively, and a hard disk space is allocated by reusing a distributed RAID algorithm. This capacity expansion method does not need to add many hard disks at a single time, and therefore can reduce the time of reconstructing the original RAID group. However, during the capacity expansion, large amounts of small block data need to be migrated, which requires a long capacity expansion time.